Scam experience in a PTC site

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It's been a long time that I haven't work online. So now I'm back to work again for I need some extra money. I checked my previous earning balance in a PTC site which I worked before, I remember that I still have $20 on that site. I was so disappointed last night as I checked my earnings, because I wanted to withdraw it. But all my earnings are gone! I look for my withdrawal and the PTC site just confirmed that they paid me last June 2011 in my Paypal account. As I open my account in paypal and search for the payment received history, there wasn't any proof of payment form that site. I know that it's not the Paypal's fault. Because all my transactions in paypal are being shown on my account. It means to say I was scammed by that PTC site. Do you have any experience with this?